Switch from gvt to dep

[Dep](https://github.com/golang/dep) is now considered ready for production.

It would be awesome to switch Hellogopher to use this instead of gvt.

Start by locating the repository's gvt configuration and invocation points, then compare them with the expected dep workflow. Replace the dependency-management setup so the project uses dep instead of gvt, and verify that the normal clone-and-make workflow still succeeds.

Scritto dal modello di indicizzazione a partire dal testo della issue.